The present invention relates to hydraulic valves, and more particularly to spool-type valve, such as hydraulic servo valves, of high dynamic response in which the spool member is moved at high velocities relative to its associated sleeve or bushing member.
Spool type valves are frequently employed in two-stage hydraulic servo valves, in which the spool position is adjusted to modulate the fluid flow through the body of the valve. These valves are quite generally known, and are described, for example in U.S. Pat. Nos. 3,103,739; 3,228,423; 3,257,911; and 4,337,797.
In these devices, there is a spool member provided with lands and bearings and a valve body, i.e., sleeve or bushing, with a generally cylindrical cavity in which the spool is slidably mounted. The lands and bearings of the spool member engage cooperating cylindrical surfaces of the cavity, and permit accurate axial movement of the lands relative to various ports in the valve body so that movement of the spool member accurately and repeatably controls the fluid flow through the valve body.
In high performance valves of this type where the spool member and the sleeve may encounter relative velocities greater than forty inches per second, the surfaces of the bearings and lands may chafe or gall on the mating cavity surfaces. This apparently occurs because of direct metal-to-metal contact between the relatively sliding members, especially in the vicinity of the bearings.
The conventional approach to this problem, particularly as described in U.S. Pat No. 4,337,797, is to provide a coating of a soft metal, such as copper, on the spool member bearing surface and elsewhere that sliding contact may be experienced. This soft material bonded to the bearing surface serves the purpose of absorbing and burying any microcontaminants that may be found in the hydraulic fluid. Providing the opposed sliding surfaces with widely separated hardnesses avoids adhesive contact such as galling between the surfaces.
Where valves of this type have been employed in a heavy-duty, high-speed, high-flow application, a high rate of failure has been experienced due to galled spools and bushings. Upon inspection of the failed valves it was discovered that there was damage to the soft copper surfaces caused by localized cavitation. The cavitation had resulted in erosion of the copper, and this permitted contact of the steel substrate with the steel surface of the valve body cavity, resulting in galling at high spool velocities.
Cavitation occurs when, because of the quick spool movement, the fluid pressure in a localized area falls below the vapor pressure of the fluid. This will result in the formation of gas bubbles within the fluid. As the fluid progresses to an area of higher pressure, the gas bubbles collapse, returning the gas into solution with the fluid. The collapse of the gas bubbles can result in pitting of the copper coating and resultant loss of bearing surfaces. High speed operation is also thought to result in localized loss of lubrication between the sliding members which could result in the attachment or welding of the parts commonly known as galling.
Because of this problem, the copper-plated bearing type spool valve is poorly suited for some applications, such as in seismographic exploration, i.e., vibrioses. In vibrioses, the typical frequency sweep of seismographic exploration vehicles exceeds 200 Hz. In one field evaluation, conventional copper-plated spools were found to fail after only four to eight hours of operation at 45 Hz.
Consequently, the industry has sought an improved valve construction which would avoid the above-noted problems attributed to cavitation and galling.